1. Field of the Invention
The present invention relates to plasma systems, and more particularly to systems in which a transfer arc is created between a plasma gun and a target in an inert atmosphere under conditions of high temperature and supersonic speeds to accomplish various tasks at the target including the deposition of a coating of material introduced at the plasma gun.
2. History of the Prior Art
It is known in the art to have a plasma system in which a plasma gun in combination with a power supply provides a transfer arc in the form of a flame of ionized gas between the gun and a workpiece or other target. The plasma gun is typically mounted within a closed container together with the target, and may be coupled to a scanning mechanism so as to direct a plasma stream onto various different portions of the target. The plasma stream acts as a conductor for ionized inert gas introduced at high temperature and which may flow through the closed container at supersonic speeds such as Mach 2 or Mach 3 in conjunction with a vacuum system coupled to the closed container to provide a transfer arc. In this manner powdered metals and similar materials introduced at the plasma gun are entrained into the plasma stream for deposition on the target. Other functions can also be achieved with such arrangements such as the melting of a member coupled as the workpiece or target and the making of metallic powders.
A common problem with plasma systems is the formation of oxides at the workpiece or target in conjunction with powder spraying, melting and other common operations. In spite of the use of a relatively pure inert gas in the formation of the transfer arc and in spite of the supersonic speeds at which the plasma stream travels, oxides still form at the workpiece or target.
Various arrangements and schemes have been devised in an attempt to remove oxides from the workpiece or target. One such arrangement which has been found to be particularly effective is described in U.S. Pat. No. 4,328,257 of Muehlberger et al which issued May 4, 1982 and which is commonly assigned with the present application. The Muehlberger et al patent describes a plasma system which includes a switching arrangement in conjunction with a direct current power supply coupled between the plasma gun and the workpiece or target so that the workpiece can be made cathodic relative to the plasma gun to create a reverse transfer arc at predetermined intervals. This creates a sputtering effect in which electrons and atoms are ejected from the workpiece despite the impacting plasma flow and the ambient pressure level. The workpiece can be rapidly heated to a working temperature, with or without a transfer arc, cleaned by the removal of atoms from the workpiece at a controlled rate during reversal of the transfer arc for a predetermined interval, and then coated, with or without an overlap between the coating and the sputtering intervals. Coating may then be completed using the transfer arc if desired.
The plasma arrangement described in U.S. Pat. No. 4,328,257 of Muehlberger et al has been found to be very effective in removing oxides from a workpiece or target and from coatings sprayed onto the workpiece or target so that a strong, well bonded coating of relatively pure material results. However, alternative arrangements and techniques for accomplishing this result would be advantageous, including in particular the ability to reduce the formation of oxides in the first instance or at least to prevent oxides which are formed from becoming a part of the workpiece or target.
Accordingly, it is an object of the invention to provide an arrangement for reducing oxides at a workpiece or target in a plasma system.
It is a further object of the invention to provide an arrangement for removing oxides as they form at a workpiece or target during melting of the target, spraying of powdered metal on the target or other operation in a plasma system.